The present invention relates to ceramic compositions and products, and more particularly, to nanometric ceramic compositions containing one or more sugars or sugar alcohols with improved physical characteristics which are useful in many industrial applications. The invention also relates to new methods for the production of nanometric ceramic compositions and products which are stronger and less prone to defects.
In the past decade, many companies have begun production of nanometric-scale powders for engineering and other industrial applications. In addition to ultrafine metal particles that are manufactured in Japan specifically for use in magnetic recording tape, nanometric powders available in industrial scale quantities have largely been carbides (WC is the most used), and the ceramic oxides (e.g., SiO2, CeO2, TiO2, Al2O3, FeO and antimony oxide) as well as others. Applications for these nanometric ceramic powders have included ultra-thin dielectrics, solid-oxide fuels, and oxygen separation systems, to name just a few.
Traditional manufacturing has involved a sequential process of (i) blending nanometric powders with organic liquids into a viscous paste, (ii) molding the paste into an engineering component (green body), (iii) pyrolizing the organic constituents by heating the green body to approximately 200xc2x0 C., and (iv) high-temperature kiln firing (sintering). During sintering, interparticle void space (porosity) in the compact is eliminated by atomic diffusion, a process causing significant shrinkage of the engineering part (e.g., 50%). This shrinkage increases as the green-body porosity increases. If the green-body porosity is above a critical value, sintering shrinkage is excessive, and this results in cracks and warpage. In addition, if the green-body porosity is not volumetrically uniform, sintering produces nonuniform shrinkage and more cracks.
Thus, a serious problem is the inability to mold pastes of nanometric powders into green bodies with low porosity and volumetrically uniform porosity. As a result, cracks and warpage during sintering represent a major obstacle towards the realization of new markets for engineering nanopowders. There are at least a few reasons why this might occur. One of these is that Van der Waals attractions cause nanoparticles to spontaneously agglomerate into porous, skeletal structures. This occurs when powders are dry or suspended in a liquid. Interparticle contact friction prevents rearrangement of these skeletal structures into less porous agglomerates. The application of mechanical pressure is not enough to compact these skeletal structures into low-porosity green-bodies. Interparticle contact friction is simply too high.
This problem can be alleviated to some extent by mixing nanoparticles with various organic chemicals (e.g., methacrylates, acrylamides, polyethylene oxide, and vinyl alcohols). However, there are serious problems with the pyrolysis of these organics: (i) slow and costly heating (e.g., 200xc2x0 C. for one week) is required to avoid the formation of cracks and gas bubbles, (ii) toxic fumes are emitted, and (iii) residual carbon contaminates the final microstructure. In many cases, large concentrations of organic solvents (e.g., alcohols, ketones, and waxes such as polyethylene wax) are needed to dilute these additives when blended with nanopowders. Safety and environmental hazards are associated with the handling, evaporation, and recycling of these solvents. It should also be recognized that capillary stresses during solvent evaporation are inversely proportional to the particle size. Extremely large capillary stresses occur during drying of green bodies made of nanopowders. These large stresses often produce cracks, which do not heal during subsequent sintering. Usually these cracks are serious enough to fracture the green body into two or more separate fragments.
What is therefore needed in the art are new nanometric ceramic compositions which, when molded from a cast and then sintered, preferably provide substantially improved ceramic bodies in which stresses leading to warps, fissures and cracks are reduced or eliminated.
According to one embodiment of the invention there is provided a ceramic composition which is made up of at least one nanometric ceramic powder, at least one lower saccharide, and water. Also contemplated is an industrial product which is manufactured using this composition.
In a further embodiment of the invention, there is provided a nanometric ceramic composition which contains about 25 to 75 vol. % of at least one nanometric ceramic powder, about 0.1 to 5 wt. % of at least one lower saccharide, about 0.001 to 1 wt % of at least one ionic material, and water.
Also provided as part of the invention is a method for reducing the viscosity of a nanometric ceramic composition in which an effective amount of at least one lower saccharide is added thereto. In addition, there is also provided a method for reducing the porosity as well as the defects of a ceramic green body which comprises adding an effective amount of at least one lower saccharide thereto.
In another aspect of the invention there is provided a method of forming a nanometric ceramic composition in which about 25 to 75 vol. % of at least one nanometric ceramic powder, about 0.1 to 5 wt. % of at least one lower saccharide, and about0.001 to 1 wt % of at least one ionic material is admixed with water so as to produce a paste or slurry.
Additional advantages and features of the present invention will become more readily apparent from the following detailed description and drawings which illustrate various embodiments of the invention.